Chromium conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Conversion coatings containing hexavalent chromium have been used for many years to passivate steel, aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys. It is primarily used as a corrosion inhibitor, primer, decorative finish, or to retain electrical conductivity.
Hexavalent chromium conversion coatings have fallen out of favor due to the toxicity of hexavalent chromium. In many locations, hexavalent chromium is highly regulated and subject to various use and handling restrictions. Hexavalent chromium conversion coatings are slowly being replaced by non-hexavalent chromium conversion coatings, particularly those utilizing trivalent chromium. Trivalent chromium conversion coatings are an environmentally friendly and superior alternative to hexavalent chromate coatings. They can be used in extreme applications to provide superior wear and corrosion resistance while also being more economical in cost per application.
The performance of various trivalent chromium coatings is sensitive to the pH of the coating solution at the time of application. If the pH of the coating solution is too high or too low, the coating does not provide the desired level of performance—e.g., with respect to corrosion resistance, paint adhesion, electronic resistance, etc. This is a problem because conventional trivalent chromium coating solutions are not pH stable; the pH of the solution has a tendency to drift over time. This also means that before each use, the pH of the solution should be checked and adjusted if it is too high or too low. Measuring and adjusting the pH before each use is a tedious process.